Quote:
Originally Posted by rmuhlack
Indeed. Providing a 100% fully illuminated field of the required size for imaging (usually greater than a 1" diameter) will necessitate a certain secondary size. Not because of optic quality but simply because of physics.
Precision optics (Vixen, Orion etc) may help to make that contrast reduction less obvious, but it cant take it away.
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Well to me precision reflector optics is something of the order of AP's Mak.
http://www.astro-physics.com/index.h...0mak/10f146mak
For a high contrast Newtonian, something like a 15% obstruction by Dia is recommended. My 6" newt has an 18% obstruction.
My 10" newt about an 21% obstruction. AP's Mak has a has 23% obstruction, which is starting to get kind largish. But I wouldn't expect my 10" to deliver more contrast than an AP Mak. In fact I'd expect the Mak to do better because of relative optical perfection in everything else.
But then a 6" newt with an extreme FL or F/12 an very average optics and a tiny obstruction can compete with the best. You could buy a 5" Tak tube for $6000 and find cheap long FL newt costing $300 could give it a run for the money.
Going back to the original issue, on the face of it the 3.5" Diag should give a 1 inch fully illuminated field. But my guess that is the nominal size, and after the bevel and secondary holder he gets only a 3.1" real Diag. This only gives 0.5" full illumination.
I suspect the nominal 4" gives him a real working 3.5".